The Polio vaccine problems was nearly 70 years ago. If it's relevant in any way, it's in an example of mistakes we made and learned from. As for the Dengue thing, it seems particularly complicated and political. There appears to be reports that the Dengvaxia vaccine has saved and estimated million lives. It's approved for sale in 19 countries. BUT political controversy and perhaps sloppy trial and study seem to have contributed to this unwarranted vaccine hesitancy in the Philippines which may have led to a measles outbreak and 400 deaths. Neither of these, btw, were issues that showed up years later. Even the dengvaxia problems, the small chance it could actually create a WORSE dengue infection, was found pretty early. They're both also related to the thing they're trying to cure. It's not like they discovered that taking dengvaxia makes you more susceptible to Coronavirus or something. That's the straw man most 'hesitant' people seem to be referring to when they argue that it was rushed. it wasn't, it's the culmination of decades of scientific research. There were people arguing that it SHOULD have been rushed even, right around when stage 2 trials finished. But all the evidence on the ground points to extreme caution and adhering to the scientific process. Even the J&J pause that was criticized by many, was overly cautious. There's really no evidence based reason to be hesitant. Maybe I'd buy it months from now, if we've actually reduced cases to almost zero, that you want to wait a year or two to vaccinate your 4 year old because it's not longer a pandemic and a large (but non-zero!) percentage of four year olds aren't really at a risk of infection, of spreading it, or long term side effects. (Not me, my kids are going to be first in line) There is almost no tangible reward in a risk/reward calculation. The risk is astronomical. Like on one hand it's "new global disease like the flu that we constantly battle for decades or more" and on the other it's "uh, maybe one day it'll make me sicker or something?" It'd be like being on a beach in the summer and deciding not to drink anything because it might make you feel bloated and then you'd sink and drown.